1. Field of the Invention
This invention relates to a method of ensuring reproducible critical dimensions of features on microcircuits by compensating for irregularities in the process of printing from a mask with an x-ray source, and in particular for features whose critical dimension is less than 250 nanometers.
2. Description of Related Art
The progression in computing power in microcircuits has been characterized by the quadrupling of the density of elements on the chip with each new generation of chip. The progression has been quite predictable, and people in the industry are accustomed to seeing semi-logarithmic plots of element density versus time. Similar plots are made for the dimension of the smallest feature in these circuits which is the gate electrode of a metal-oxide-silicon chip. The critical dimension determines the design rule, or generation of a new chip. For example, a 0.5 micron design rule is typical of a 16 MB DRAM, a 0.35 micron design rule typifies a 64 MB DRAM, a 0.25 micron design rule typifies a 256 MB DRAM, and so on. The critical dimension of each generation decreases by a factor of about 1.4 to quadruple the memory.
Lithographic methods have improved to provide expanded computing power, moving from visible light, to ultraviolet light, to x-ray energy sources which expose the photoresist which will define the gate electrode. All this was done to keep the wavelength exposing the resist less than the critical dimension in the generation. This effort is made to avoid diffraction effects which would unfaithfully reproduce the dimension of a feature in the mask upon the photoresist on the chip.
The error budget for faithfully reproducing critical dimension across a 3 cm by 3 cm field becomes unacceptably large for features which are 0.25 micron or less in width. Several effects contribute to the variability of etched features. The severest effect is global divergence, which is caused by the divergence of energy from the source of rays which is not collimated. The feature directly below the source is faithfully reproduced upon the resist covering the layer to be etched, but a feature at the edge of the field, where the exposing rays are not perpendicular to the mask, is reproduced with a trapezoidal cross section in near-contact printing operation. The gate electrode is typically etched in a reactive ion etching chamber in which a bias applied to the wafer causes ions to impinge normally upon the resist cross-section. The vertical projection of the trapezoidal cross section upon the gate electrode material is larger than that at the center of the exposure field. This effect can cause a 10 nanometer (nm) change for a 250 nm design rule line. Of course, this effect could be minimized if the source to mask distance was increased, or if the exposure field was reduced. But the intensity of the radiation decreases as the square of the distance to the mask, so throughput would be substantially lowered. Reducing the exposure field would constrain the layout of the chip.
Another factor in the error budget is local divergence, which is caused by the finite dimension of the x-ray source. This causes a penumbra to be cast upon the resist below the mask feature, and unfortunately, this penumbra may vary across the exposure field, if a collimating optic is used. The effect can contribute a plus or minus 4 nm variation to a 250 nm line.
A final factor is dose non-uniformity. Areas of the photoresist which are not directly under the source receive less energy per unit area than an area which is closest to the source and which receives perpendicular rays. This factor can also contribute a plus or minus 4 nm error to a 250 nm line.
Accordingly, there is an increased need in the art to control these dimensions in 250 nm design rule features and in the next generation for 180 nm features across a reasonably sized field with a source to mask distance which will provide a reasonable throughput in the wafer fabrication process. It would be desirable to compensate for these effects in making the x-ray mask so that the features which are ultimately etched on the chip are correctly sized and uniformly distributed. It would also be desirable to avoid the use of collimating x-ray optics which are not very efficient and which are difficult to make.